This invention relates to load pull testing of VHF frequency transistors and amplifiers, at frequencies typically between 10 MHz and 150 MHz, under high power operating conditions, using automatic impedance tuners in order to synthesize impedances at the input and output of the test devices (DUT) at the fundamental and various harmonic frequencies.
Load Pull is a measurement technique, in which the source or load impedance of a DUT (typically a power RF transistor) is changed systematically using pre-calibrated impedance tuners. This technique is popular at frequencies in the GHz frequency range but unknown in the VHF frequency range, especially below 100 MHz.
Accurate design of high power amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (RF transistor's) characteristics under high power operation conditions. In such circuits, it is insufficient and inaccurate to describe transistors, operating at high power in their highly non-linear regions close to saturation, using analytical or numerical models only. Instead the transistors need to be characterized using specialized test setups under the actual operating conditions.
A popular method for testing and characterizing such components (transistors) under high power operation conditions has been developed at much higher frequencies in the GHz frequency range, as “load pull” and “source pull”. Load pull or source pull are measurement techniques employing RF impedance tuners and other RF test equipment, such as RF signal sources and RF power meters. Since transistors are typically used close to power saturation conditions in high efficiency amplifiers, their internal nonlinearities create harmonic frequency components. Those components, if not presented with the appropriate RF impedance will degrade the performance of the amplifier. In order to determine the proper RF impedance a Harmonic Load Pull system is required. In such a system certain components such as frequency discriminators (Diplexers or Triplexers) are used, whose task is to separate the harmonic components generated by the DUT into different paths, where they can be treated separately (FIG. 1). Harmonic impedance tuners [4] are also used in order to manipulate the harmonic microwave impedances presented to and under which the Device under Test (DUT, amplifier or transistor) is tested (FIG. 2).
FIG. 1 shows a setup using a frequency diplexer or triplexer in order to create three independent frequency paths, loaded each with a wideband tuner. This concept of harmonic tuning is well known in the literature and is valid for all frequencies for which appropriate components such as tuners and diplexers/triplexers exist. Because of lack of such components harmonic load pull test systems are not known for frequencies below 800 MHz. This, on the other hand is due to the difficulty in manufacturing such components because of their required size to handle low frequencies and large wavelengths.
A simpler solution for a harmonic load pull test system will use a single multi-harmonic tuner [7], i.e. a tuner which allows independent tuning at harmonic frequencies without using frequency discriminators, such as diplexers and triplexers. Such a setup is shown in FIG. 2.